Abstract:

Disclosed herein is a method and apparatus for reducing a nitrogen oxide,
and the control thereof.

Claims:

1. An apparatus for reducing a nitrogen oxide gas emitted by a emissions
source, comprising(a) an exhaust conduit for transporting the nitrogen
oxide gas downstream from the emissions source,(b) an injector for
injecting a reducing agent into the conduit, and(c) one or more gas
analyzers located in the conduit upstream from the injector.

2. An apparatus according to claim 1 further comprising a catalyst to
catalyze the reduction of the nitrogen oxide.

3. An apparatus according to claim 2 further comprising a gas analyzer
that is downstream from the injector and downstream from a catalyst.

4. An apparatus according to claim 2 wherein a first gas analyzer is
located upstream from a catalyst, and a second gas analyzer is located
downstream from the catalyst.

5. An apparatus according to claim 2 that comprises a plurality of gas
analyzers, wherein a plurality of gas analyzers is located upstream from
a catalyst, and a plurality of gas analyzers is located downstream from
the catalyst.

6. An apparatus according to claim 2 comprising (a) a first catalyst, (b)
a gas analyzer located downstream from the first catalyst, and (c) a
second catalyst located downstream from the gas analyzer.

7. An apparatus according to claim 6 wherein the first and second
catalysts is each a catalyst bed in vertical arrangement.

8. An apparatus according to claim 6 further comprising a plurality of gas
analyzers located between the first and second catalysts.

9. An apparatus according to claim 6 further comprising one or more gas
analyzers downstream from the second catalyst.

10. An apparatus according to claim 2 further comprising one or more gas
analyzers downstream from all catalysts.

11. An apparatus according to claim 1 or 2 wherein the reducing agent is
ammonia.

12. An apparatus according to claim 1 or 2 wherein the reducing agent is
urea.

13. An apparatus according to claim 1 or 2 wherein the emissions source is
stationary.

14. An electrical generating plant comprising an apparatus for reducing a
nitrogen oxide gas according to claim 1 or 2.

15. A furnace comprising an apparatus for reducing a nitrogen oxide gas
according to claim 1 or 2.

16. A steam turbine comprising an apparatus for reducing a nitrogen oxide
gas according to claim 1 or 2.

17. A gas turbine comprising an apparatus for reducing a nitrogen oxide
gas according to claim 1 or 2.

18. A vehicle for transportation or recreation comprising an apparatus for
reducing a nitrogen oxide gas according to claim 1 or 2.

19. A piece of equipment for construction, maintenance or industrial
operations comprising an apparatus for reducing a nitrogen oxide gas
according to claim 1 or 2.

20. In a multi-component gas mixture that is emitted by a emissions source
and contains a nitrogen oxide, wherein a nitrogen oxide is reduced by
injecting a reducing agent into the gas mixture, a method of decreasing
the amount or release of unreacted reducing agent comprisingdetermining
information as to the compositional content of the gas mixture,
andcontrolling the injection of the reducing agent in relation to the
information as to the compositional content of the gas mixture.

21. A method according to claim 20 wherein the gas mixture is contacted
with a catalyst, and information as to the compositional content of the
gas mixture is determined before the gas mixture contacts any catalyst.

22. A method according to claim 21 further comprising a step of
determining information as to the compositional content of the gas
mixture after the gas mixture contacts any catalyst.

23. A method according to claim 20 wherein the gas mixture is contacted
with a catalyst, and information as to the compositional content of the
gas mixture is determined after the gas mixture contacts any catalyst.

24. A method according to claim 20 wherein the gas mixture is contacted
with first and second catalysts, and information as to the compositional
content of the gas mixture is determined after the gas mixture contacts a
first catalyst but before the gas mixture contacts a second catalyst.

25. A method according to claim 20 wherein the gas mixture is contacted
with a catalyst, and information as to the compositional content of the
gas mixture is determined after the gas mixture contacts all catalyst.

26. A method according to claim 20, 21, 23 or 25 further comprising a step
of determining the amount of reducing agent to be injected into the gas
mixture in relation to the information as to the compositional content of
the gas mixture.

27. A method according to claim 20, 21, 23 or 25 wherein the information
as to the compositional content of the gas mixture is an output of one or
more gas analyzers.

28. A method according to claim 27 wherein the gas mixture is transported
downstream from the emissions source by an exhaust conduit, and a gas
analyzer located in the conduit.

29. A method according to claim 20, 21, 23 or 25 wherein the information
as to the compositional content of the gas mixture is determined from an
array of chemo/electro-active materials.

30. A method according to claim 20, 21, 23 or 25 wherein the information
as to the compositional content of the gas mixture is related to the
individual concentration within the gas mixture of an individual gas
component therein.

31. A method according to claim 20, 21, 23 or 25 wherein the information
as to the compositional content of the gas mixture is related to the
collective concentration within the gas mixture of a subgroup of the
component gases therein.

32. A method according to claim 20, 21, 23 or 25 wherein the information
as to the compositional content of the gas mixture is related to the
individual concentration within the gas mixture of an individual gas
component therein, and is related to the collective concentration within
the gas mixture of a subgroup of the component gases therein.

33. A method according to claim 20, 21, 23 or 25 wherein the information
as to the compositional content of the gas mixture is inputted to a
decision-making routine.

34. A method according to claim 20, 21, 23 or 25 wherein the information
as to the compositional content of the gas mixture is inputted to a map.

35. A method according to claim 20, 21, 23 or 25 wherein the information
as to the compositional content of the gas mixture is related to the
individual concentration within the gas mixture of an individual nitrogen
oxide component therein.

36. A method according to claim 20, 21, 23 or 25 wherein the information
as to the compositional content of the gas mixture is related to the
collective concentration within the gas mixture of all nitrogen oxide
components therein.

37. A method according to claim 27 wherein a gas analyzer comprises an
array of chemo/electro-active materials.

38. A method according to claim 27 wherein a gas analyzer outputs at least
one signal that is related to the individual concentration within the gas
mixture of an individual gas component therein.

39. A method according to claim 27 wherein a gas analyzer outputs at least
one signal that is related to the collective concentration within the gas
mixture of a subgroup of the component gases therein.

40. A method according to claim 27 wherein a gas analyzer outputs at least
one signal that is related to the individual concentration within the gas
mixture of an individual gas component therein, and at least one signal
that is related to the collective concentration within the gas mixture of
a subgroup of the component gases therein.

41. A method according to claim 27 wherein a gas analyzer outputs a signal
to a decision-making routine.

42. A method according to claim 27 wherein a gas analyzer that is upstream
from all catalyst, and a gas analyzer that is downstream from all
catalyst, both output a signal to a decision-making routine.

43. A method according to claim 27 wherein a gas analyzer that is upstream
from all catalyst, a gas analyzer that is downstream from a first
catalyst and upstream from a second catalyst, and a gas analyzer that is
downstream from all catalyst, each outputs a signal to a decision-making
routine.

44. A method according to claim 27 wherein the gas analyzer outputs a
signal to a map.

45. A method according to claim 27 wherein a gas analyzer outputs a signal
to a decision-making routine that controls the injection of reducing
agent.

46. A method according to claim 27 wherein a gas analyzer outputs a signal
to a decision-making routine that calculates an amount of reducing agent
to be injected.

47. A method according to claim 27 wherein the gas analyzer outputs at
least one signal that is related to the individual concentration within
the gas mixture of an individual nitrogen oxide component therein.

48. A method according to claim 27 wherein the gas analyzer outputs at
least one signal that is related to the collective concentration within
the gas mixture of all nitrogen oxide components therein.

49. A method according to claim 27 wherein a gas analyzer outputs at least
one signal that is related to the individual concentration within the gas
mixture of one or more or all of the nitrogen oxide component(s) therein,
and the signal is outputted to a decision-making routine that calculates
an amount of reducing agent to be injected.

50. A method according to claim 20, 21, 23 or 25 wherein the emissions
source is stationary.

51. A method according to claim 20, 21, 23 or 25 wherein the emissions
source is a vehicle for transportation or recreation or a piece of
equipment for construction, maintenance or industrial operations.

Description:

[0001]This application claims the benefit of U.S. Provisional Application
No. 60/389,781, filed on Jun. 19, 2002, which is incorporated in its
entirety as a part hereof for all purposes.

FIELD OF THE INVENTION

[0002]This invention relates to methods and apparatus for reducing a
nitrogen oxide. In particular, it relates to the use of a gas analyzer to
obtain information related to the compositional content of a
multi-component gas mixture that contains a nitrogen oxide for the
purpose of assisting in the control of the reduction.

TECHNICAL BACKGROUND

[0003]Oxides of nitrogen (NOX) that are emitted by an emissions
source, such as those formed as a result of combustion, are included
among the main causes of the "acid rain" problem, the photochemical smog
problem and the resulting damage to the environment. These harmful
substances should therefore be eliminated to the greatest extent possible
from the gases emitted by an emissions source, such as the exhaust from a
combustion process, prior to their discharge into the atmosphere.

[0004]One source of nitrogen oxides, in the form of NO2 and mainly
NO, are those formed by the combustion of coal, oil, gas, gasoline,
diesel fuel or other fossil fuels. Combustion of fossil fuels occurs, for
example, in a stationary device such as furnace, which is a device for
the production or application of heat. A furnace may be used in
connection with a boiler such as in a steam generator that drives a steam
turbine in an electrical generating plant, in connection with an
industrial operation such as in a smelter or chemical reactor, or in
connection with supplying heat for human consumption.

[0005]Fossil fuels are also combusted in a mobile device, including a
device that supplies mechanical power such as an internal combustion
engine in a vehicle for transportation or recreation, or in a piece of
equipment for construction, maintenance or industrial operations; or in a
gas turbine, which is a turbine driven by a compressed, combusted fluid
(such as air), such as in the engine of a jet aircraft. Gas-emitting
devices such as an internal combustion engine or a gas turbine are also
found in stationary applications, however. The exhaust gas emitted by
devices such as those described above is a multi-component mixture of
gases containing nitrogen oxides. Nitrogen oxides are also emitted by
plants for the incineration of industrial or municipal waste. In
addition, carbon monoxide and hydrocarbons are also emitted by these
sources.

[0006]A problem exists with respect to the need for control of the
injection of a reducing agent into a gas mixture containing nitrogen
oxides. There is a desire to effect the reduction of as large a quantity
of the nitrogen oxides present in the gas mixture as possible. For this
purpose, what amounts to a stoichiometric excess of reducing agent, in
terms of the quantity of nitrogen oxides present, is often injected into
the gas mixture and thus into the nitrogen oxides. An excess of reducing
agent is employed not so much by design but primarily because of the
unavailability of information related to the compositional content of the
gas mixture sufficient to accurately calculate the stoichiometric
equivalent of reducing agent needed. The compositional content of a gas
mixture containing nitrogen oxides often varies in an extremely
unpredictable manner as it moves through a conduit from its emission
source to the point of its ultimate destination, such as a point of
discharge into the atmosphere. As a result, because of the desire to
obtain reduction of a large percentage of the nitrogen oxides, an amount
of reducing agent is injected that later proves to be an excess. Whether
this results from calculations based on inaccurate or incomplete
information, a strategy of employing an excess to be certain that too
little is not employed, or incomplete reaction of whatever the amount,
the same undesired consequence is experienced--unreacted reducing agent
is discharged to the atmosphere and becomes a pollutant itself. When
ammonia is the reducing agent, this is known as ammonia slip. In a gas
mixture that is unscrubbed, or otherwise contains sulfur oxides,
unreacted ammonia is also capable of reacting with the sulfur oxides to
yield corrosive, sticky deposits of ammonium sulfate and/or ammonium
hydrogen sulfate that foul the mechanism of the conduit.

[0007]There is a need then for a method and apparatus for the reduction of
a nitrogen oxide that provides control of the reaction of reduction, and
in particular control of the injection of a reducing agent into the gas
mixture containing the nitrogen oxide. In particular, there is a need for
a method and apparatus that enables the calculation of the amount of
reducing agent to be injected in relation to information about the
compositional content of the gas mixture.

[0008]This invention addresses those needs by providing a method and
apparatus in which analysis of the gas mixture is performed to furnish
information related to the compositional content thereof. In certain
embodiments, the analysis is furnished by a gas analyzer that may be
placed within a conduit through which the gas mixture is transported in
positions that create an opportunity to develop useful information about
the gas mixture, and especially information related to the nitrogen oxide
content thereof. In certain other embodiments, a gas analyzer is employed
for this purpose that outputs a signal related to the content within the
gas mixture of an individual component gas therein and/or the collective
content of a sub-group of gases therein. In certain other embodiments,
the information is inputted into a decision making routine and/or a map,
and may be used to calculate a desired amount of reducing agent to be
injected into the gas mixture, and thus into the nitrogen oxides to be
reduced. Other embodiments of the invention are as more particularly
described below, or are as would be apparent to the artisan in view of
the description below.

SUMMARY OF THE INVENTION

[0009]One embodiment of this invention is an apparatus for reducing a
nitrogen oxide gas emitted by a emissions source that involves (a) an
exhaust conduit for transporting the nitrogen oxide gas downstream from
the emissions source, (b) an injector for injecting a reducing agent into
the conduit, and (c) one or more gas analyzers located in the conduit
upstream from the injector.

[0010]Another embodiment of this invention, in a multi-component gas
mixture that is emitted by a emissions source and contains a nitrogen
oxide, wherein a nitrogen oxide is reduced by injecting a reducing agent
into the gas mixture, is a method of determining the amount of reducing
agent to be injected, or of decreasing the amount or release of unreacted
reducing agent, by determining information as to the compositional
content of the gas mixture, and controlling the injection of the reducing
agent in relation to the information as to the compositional content of
the gas mixture.

[0011]Another embodiment of this invention, in a multi-component gas
mixture that is emitted by a emissions source and contains a nitrogen
oxide, wherein a nitrogen oxide is reduced by injecting a reducing agent
into the gas mixture and contacting the gas mixture with a catalyst, is a
method of determining the amount of reducing agent to be injected, or of
decreasing the amount or release of unreacted reducing agent by
determining information as to the compositional content of the gas
mixture before the gas mixture contacts any catalyst, and controlling the
injection of the reducing agent in relation to the information as to the
compositional content of the gas mixture.

[0012]Another embodiment of this invention, in a multi-component gas
mixture that is emitted by a emissions source and contains a nitrogen
oxide, wherein a nitrogen oxide is reduced by injecting a reducing agent
into the gas mixture and contacting the gas mixture with a catalyst, is a
method of determining the amount of reducing agent to be injected, or of
decreasing the amount or release of unreacted reducing agent by
determining information as to the compositional content of the gas
mixture after the gas mixture has contacted a catalyst, and controlling
the injection of the reducing agent in relation to the information as to
the compositional content of the gas mixture.

[0013]Another embodiment of this invention, in a multi-component gas
mixture that is emitted by a emissions source and contains a nitrogen
oxide, wherein a nitrogen oxide is reduced by injecting a reducing agent
into the gas mixture and contacting the gas mixture with a catalyst, is a
method of determining the amount of reducing agent to be injected, or of
decreasing the amount or release of unreacted reducing agent, by
determining information as to the compositional content of the gas
mixture after the gas mixture has contacted all catalyst, and controlling
the injection of the reducing agent in relation to the information as to
the compositional content of the gas mixture.

DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 depicts an array of chemo/electro-active materials.

[0015]FIG. 2 is a schematic of the pattern of interdigitated electrodes
overlaid with a dielectric overlayer, forming sixteen blank wells, in an
array of chemo/electro-active materials.

[0017]FIG. 4 is a schematic layout of the flow of a gas, such as the
combustion exhaust from a boiler, through an SCR system.

[0018]FIG. 5 is a schematic layout of the flow of a gas, such as the
combustion exhaust from a boiler, through an SCR system.

[0019]FIG. 6 shows the placement of a catalyst or a catalyst bed in an SCR
system.

[0020]FIG. 7 is a schematic layout of the flow of a gas, such as the
combustion exhaust from a boiler, through an SCR system containing a gas
analyzer.

[0021]FIG. 8 is a schematic diagram of an internal combustion engine
showing the placement of a gas analyzer.

[0022]FIG. 9 is a schematic diagram of an internal combustion engine
showing the placement of a gas analyzer in connection with an SCR system.

DETAILED DESCRIPTION OF THE INVENTION

[0023]Nitrogen oxides may be reduced by contact with a reducing agent in
the absence of a catalyst at a temperature of about 850 to about
1200° C., preferably about 900 to about 1100° C. This is
usually referred to as selective non-catalytic reduction. The most common
way of providing a temperature high enough to perform the reduction is to
inject the reducing agent into the gas mixture that contains the nitrogen
oxides in or near the source, such as a source of combustion, from which
the nitrogen oxides are being emitted. The nitrogen oxides are
predominantly transformed by the high temperature of the source of
emissions to molecular nitrogen, which is nontoxic. Ammonia (e.g.
anhydrous ammonia) is a reducing agent typically used, but urea is an
alternative choice as a reducing agent. Three to four times as much
reducing agent is required in a non-catalytic reduction, as compared to a
catalytic reduction (described below), to achieve the same extent of
reduction.

[0024]More common, then, is selective catalytic reduction, in which
diminution of the nitrogen oxide emitted by an emissions source, such as
a source of combustion, takes place through contact of the nitrogen oxide
and the reducing agent with a catalyst. In order to ensure an optimal
utilization of the needed reducing agent, selective catalytic reduction
processes are preferred for the removal of nitrogen oxides from emissions
sources such as a combustion exhaust because of the oxygen content in the
exhaust gas. As a reducing agent, ammonia gas (e.g. anhydrous ammonia)
has proven itself to be suitable because it reacts easily with oxides of
nitrogen in the presence of an appropriate catalyst for the reaction, but
only to a slight extent with the oxygen present in the gas. Urea is an
alternative choice as a reducing agent.

[0025]For the selective reduction of the nitrogen oxides contained in
combustion exhaust gases, for example, it is known to feed into the
exhaust gas stream vaporous ammonia (NH3) under pressure, or ammonia
dissolved in water, without pressure, while an effort is made, by means
of a mixing section with appropriate baffling within the adjoining
conduit gas passages, to achieve a streamer-free distribution of ammonia
and temperatures in the flow of exhaust gas. The gas mixture emitted from
a furnace flue may contain, for example, 1-20 percent by volume O2,
40 to 2000 ppm by volume nitrogen oxides, and 10 to 5000 ppm by volume
SO2 and SO3. The catalytic reduction of the nitrogen oxides by
use of ammonia as a reducing agent typically proceeds according to one or
more of these reactions:

4NO+4NH3+O2→4N2+6H2O I

2NO2+4NH3+O2→3N2+6H2O II

6NO+4NH3→5N2+6H2O III

6NO2+8NH3→7N2+12H2O IV

NO+NO2+2NH3→2N2+3H2O V

[0026]As shown in FIG. 4, in a typical combustion process, flue gases
emerging from a furnace (1) pass through a pipe (20) into a hot operating
electrofilter (2) where they are freed of dust. An ammonia/air-mixture is
then introduced into contact with the gases through injector (3), and is
distributed homogeneously in the flow of the exhaust gas downstream from
the filter (2). The mixture is then fed through pipe (22) into a
catalytic reduction reactor (4).

[0027]It is shown in FIG. 4 that the catalyst (7) in the reactor (4) may
be a vertical array of catalyst beds, a first series of beds (5) being
positioned above a second series of beds (6). It is possible, if desired,
to position a gas analyzer between individual catalyst beds, or between
the first and second series of beds 5, 6. The catalyst may be in the
form, for example, of monolithic, ceramic honeycomb catalysts disposed
one behind the other to obtain the catalytic reduction of nitrogen oxide
in the exhaust gas. There is a broad range for the permissible distances
between the catalysts, or between the individual catalyst beds, located
in the reactor (4). The dimensions of the spacing arrangement of the
catalysts or catalyst beds are determined to insure the production of a
turbulent transverse movement of gas in the conduit and avoidance of
local mixing or "channeling".

[0028]From the reduction reactor (4), the gas mixture may, if desired, be
transported through pipe 24 to a sulfur oxide scrubber (8) wherein sulfur
oxide is reacted with water or dilute aqueous sulfuric acid to form
concentrated H2SO4. The completely purified exhaust gas leaving
the scrubber (8) may then be transported by pipe (26) to chimney (9) for
discharge into the atmosphere. In FIG. 4, the exhaust is emitted from its
source, the furnace (1), and is transported through piping and other
components to its ultimate destination, the chimney (9), for discharge
into the atmosphere. The direction of flow from the furnace (1) to the
chimney (9) is considered to be downstream, and the opposite direction is
considered to be upstream. The piping and other components through which
the exhaust gas mixture is transported, and in which the reaction of
reduction occur, together provide a conduit for the flow, transport,
handling and disposition of the gas mixture. A gas analyzer, or the gas
sensing component(s) thereof, can be positioned at any location along
this conduit, whether in a pipe or within a component such as the
catalyst (7) located in reactor 4. Multiple catalyst beds are illustrated
in the apparatus of FIG. 4, and in similar fashion, the apparatus may
contain a plurality of catalysts as well.

[0029]Alternatively, as shown in FIG. 5, a dust filter (2) may be located
downstream from a catalyst (7). In a further alternative, as shown in
FIG. 6, a gas mixture to be denitrified may pass horizontally through a
reactor 30 containing one or more catalysts or catalyst beds. As
described above, multiple catalysts and/or catalyst beds may be employed
in this horizontal configuration, and one or more gas analyzers may be
located between each of the catalysts and/or catalyst beds.

[0030]In the method according to the invention, essentially all catalysts
may be used which are suitable for the selective reduction of nitrogen
oxide. Examples of these are activated carbon, or catalysts that are
mixtures of the oxides of iron, titanium (e.g. a manganese-based
TiO2), tungsten, vanadium and molybdenum (see, for example, DE 24 58
888, which is incorporated in its entirety as a part hereof for all
purposes) or catalysts formed of natural or synthetic aluminum silicates,
for example, zeolites (ZSM-5), or catalysts which contain precious metals
of the platinum group. For example, a flue gas stream containing nitrogen
oxides and sulphur oxides may be passed through a catalyst bed containing
a catalyst consisting essentially of 3 to 15% by weight vanadium
pentoxide (V2O5) on a carrier consisting of titanium dioxide
(TiO2), silica (SiO2), and/or alumina (Al2O3).

[0031]The catalyst for nitrogen oxide reduction may be of any geometrical
shape, such as in the form of a honeycomb monolith or in pellet or
particulate form. However, a catalyst shape resulting in a large void and
with parallel gas channels in the catalyst bed, such as a honeycomb
catalyst, is preferred since the conduit gas often contains considerable
amounts of dust which otherwise might clog the catalyst bed. The
honeycomb form offers lower back pressure and a simpler possibility for
cleaning off dust. A denitrification catalyst could be made for example
as a carrier catalyst consisting of mullite honeycomb bodies of the
dimensions 150 mm×150 mm×150 mm length with a cell density of
16/cm2 and a zeolite coating of the mordenite type. A moving bed is
typically used for granular activated carbon.

[0032]The catalyst can consist completely of a catalytically active mass
(solid catalyst), or the catalytically active substance can be deposited
on an inert, ceramic or metallic body, which optionally can be coated in
addition with a surface area enlarging oxide layer (carrier catalyst).
For example, the catalyst may be in the form of a solid-bed reactor with
a flow directed preferably vertically downward. The reactor may contain a
honeycomb structure, which has a crystalline vanadium-titanium compound
as the catalytically active substance. The pressure loss in the solid-bed
reactor is taken into account in establishing the size of the conduit gas
blower. The vertically downward flow in the reactor is intended to combat
the depositing of solid impurities within the catalyst or keep them
within acceptable ranges. The incrustation that occurs is removed
discontinuously by blasting with compressed air or steam.

[0033]The catalytic reaction, preferably carried out in a single reactor,
may be operated in the temperature range of about 250-550° C.,
preferably about 350-450° C., and more preferably about
380-420° C. The temperature should not be so high that the
reducing agent is degraded (as in the conversion, for example, of ammonia
into NOx and water), or so low that the reducing agent does not fully
react with the emitted NOx, is released into the atmosphere and becomes a
pollutant itself. The molar ratio of reducing agent to nitrogen oxides is
typically in the range of about 0.6-1.8, and preferably about 1.0-1.4. In
the case of a full load operation in a facility containing a combustion
source such as an electrical generating plant, a flue gas temperature of
350-400° C. may be easily reached, and these are temperatures at
which denitrification catalysts can be utilized. In the case of a
variable load operation, the flue gas temperature drops as a rule below
the minimum required for the operation of the catalyst in the partial
load area, so that a bypass connection system is typically necessary for
the branching off of flue gas before the last step of heat removal in the
boiler in order to maintain the reaction temperature.

[0034]Operations that are carried out in the zone of high dust lead,
moreover, to catalyst abrasion by the conduit dust, and may cause
deposits and thus plugging up of the catalyst channels or pores. To
prevent such complications, a cleaning by blowing off with (for example)
hot steam is required at relatively short time intervals. It is
preferred, however, that the reduction step be carried out using an
exhaust gas which has little dust content or from which the dust has been
largely removed because the mechanical and thermal load of the catalyst
is considerably less. For the removal of the dust, the use of a high
temperature electrofilter is particularly suitable. A filter of this type
requires slightly higher investments in comparison to a cold operating
electrofilter, but reheating measures and problems which are connected
with the catalyst abrasion are avoided. Both embodiments in addition have
the advantage that the removal dust is not contaminated with reducing
agent.

[0035]To obtain an efficient decrease in the content of the nitrogen
oxides in a flue gas, one approach as noted above, has been to add
reducing agent in excess of the stoichiometric amount needed according to
reactions I-V. If the reducing agent is not completely converted in the
denitrification reaction, however, and a small quantity of it (designated
as "ammonia slip" if the reducing agent is ammonia) is present in the
exhaust gas after it is emitted into the atmosphere, the usual goal of
limiting the content of reducing agent in treated flue gas to an
acceptable level, such as 5-10 ppm by volume, will not be met. The
alternative of utilizing less than stoichiometric amounts of reducing
agent, and compensating by the use of increased volumes of catalyst, will
increase the catalyst costs. The efficiency of the denitrification
process will, moreover, be decreased as the absence of a stoichiometric
amount of reducing agent will be the limiting factor in the reaction, and
reduction of nitrogen oxides at an acceptable level will not occur. The
methods and apparatus of this invention are used to furnish information
about the compositional content of the gas mixture being subjected to
denitrification to enable determination of the correct amount of reducing
agent to be injected into the gas mixture, thereby decreasing the release
of unreacted reducing agent.

[0036]For the purpose of controlling the denitrification reaction, it is
also desirable to evaluate the success of the reaction by determining
information about the compositional content of the gas mixture before it
is emitted into the atmosphere. This type of determination may be made,
for example, at one or more positions after the gas mixture has passed
the point of injection of the reducing agent, if the reaction is
uncatalyzed, or after the gas mixture has passed downstream from a
reducing reactor if the reaction is catalyzed. Alternatively, if an
oxidation catalyst is provided to oxidize unreacted reducing agent, the
compositionally-related information may be determined at one or more
positions after the gas mixture has passed downstream from the oxidation
catalyst.

[0037]When such an oxidation catalyst is employed, and the reducing agent
is for example, ammonia, ammonia is oxidized to nitrogen and water
according to the following reaction:

4NH3+3O2→6H2O+2N2 VI

Typical oxidation catalysts for this purpose are based on transition
metals, for example those containing oxides of copper, chromium,
manganese and/or iron. A catalyst consisting essentially of about 2 to 7%
by weight vanadium promoted with at least one alkali metal in a vanadium
to alkali metal atomic ratio in the range from about 1:2 to 1:5 on a
silica carrier is advantageously employed since this catalyst gives a
high degree of conversion according to the reaction VI. The alkali metal
employed is preferably potassium.

[0038]One example of the manner in which the methods and apparatus of this
invention can be used to control the reduction of a nitrogen oxide is to
control the injection of the reducing agent into the nitrogen oxide, such
as by controlling the injection of the reducing agent into a gas mixture
that contains a nitrogen oxide. In the case of nitrogen oxide that is
emitted by a source of combustion, control of the reduction reaction may
be effected in terms of the compositional content of the stream of
exhaust gas given off by the combustion. Information may be obtained that
is related to the compositional content of the exhaust gas at points in
time both before and after a reducing agent has been injected into the
nitrogen oxide.

[0039]Information related to the compositional content of a gas mixture
containing a nitrogen oxide may be obtained from a gas analyzer that is
exposed to the gas mixture. This is most conveniently done by placing one
or more gas analyzers in a conduit in which the mixture containing the
nitrogen oxide is transported from its source of emission to its eventual
destination, such as discharge into the atmosphere. In the case of
exhaust gas emitted from a source of combustion, this represents a
challenge because combustion exhaust gases reach high temperatures that
will degrade the materials and instrumentation from which many analytical
devices are made. A gas analyzer as used in this invention is one that is
not degraded by, or does not malfunction as a result of exposure to, a
gas or gas mixture having a temperature of about 300° C. or more.
Preferably the analyzer is not degraded or does not malfunction at even
higher temperatures such as about 400° C. or more, about
500° C. or more, about 600° C. or more, about 700°
C. or more, about 800° C. or more, about 900° C. or more,
or about 1000° C. or more. The gas analyzer used in this
invention, including the reactive or gas sensing components thereof, may
thus be positioned in a gas mixture having a temperature as described
above, and may thus be located in the same conduit in which the reducing
agent is injected to effect the reduction reaction. Although the analyzer
as it is installed in the conduit is connected to conductors that
transmit signal outputs of the analyzer elsewhere for further processing,
the only contact between the analyzer and the nitrogen oxide to be
reduced, or the gas mixture containing the nitrogen oxides occurs in the
conduit in which the nitrogen oxides are transported from their source to
their eventual destination. The analyzer is not operated by withdrawing
gas from the conduit for analysis in a separate chamber that is outside
of the conduit.

[0040]A gas analyzer that is exposed to a gas mixture containing a
nitrogen oxide is used to provide information related to the
compositional content of the gas mixture for the purpose of controlling
the reduction reaction. The information is used, in particular to control
the injection of the reducing agent into the nitrogen oxide, such as by
controlling the injection of the reducing agent into the gas mixture
containing the nitrogen oxide. Information as to the compositional
content of the gas mixture obtained before the reducing agent has been
injected, or before the gas mixture has contacted a catalyst (if a
catalyst is used), may be used to assist in the calculation of a
stoichiometrically correct amount of reducing agent. This
"stoichiometrically correct" amount is an amount that is sufficient to
react with all nitrogen oxides present in the mixture without providing
an excess of reducing agent that will be transported downstream with the
mixture as a pollutant itself. Information as to the compositional
content of the gas mixture obtained after the reducing agent has been
injected may be used to evaluate the accuracy of the calculation by which
the stoichiometrically correct amount of reducing agent is determined. If
it appears that the calculation is not accurate because the gas mixture
downstream from the injector, and downstream from the catalyst if a
catalyst is used, contains more nitrogen oxide than desired or more
reducing agent than desired, adjustments can be made to the calculation
in view of such information obtained downstream from the position of the
reduction reaction.

[0041]FIG. 7 shows a schematic layout of one possible placement of a gas
analyzer both upstream 40 and downstream 42 from the position of a
reduction reactor 44 in which a catalyst is employed, also upstream 46
from the point of injection of the reducing agent. By conductors 48, 50
and 52, information about the compositional content of the gas mixture is
fed to a reducing agent control system 54. In addition to a pump for
injecting the reducing agent, the reducing agent control system may
contain a decision-making routine and/or a map. Information from gas
analyzer 46 may be fed forward to control system 54 to assist in
performing a first calculation of the amount of reducing agent to be
injected into the gas mixture. Information from gas analyzer 40 may be
fed back to control system 54 to evaluate whether the reducing agent is
in place in the gas mixture to the extent and with the distribution as
desired, and, in view of such finding, to also assist in performing
adjustments as needed on the original calculation of the amount of
reducing agent to be injected into the gas mixture. Information from gas
analyzer 42 may be fed back to control system 54 to evaluate whether
nitrogen oxide and the reducing agent are both absent from the gas
mixture to the extent desired, and, in view of such finding, to also
assist in performing adjustments as needed on the original calculation of
the amount of reducing agent to be injected into the gas mixture.

[0042]The gas source 56 could be a stationary source of combustion, such
as a furnace or a boiler for a steam turbine; a source of combustion that
can be stationary, mobile or self-propelled such as a gas turbine or an
internal combustion engine; or a chemical reaction that does not involve
combustion such as an industrial process. Although ammonia is shown as
the reducing agent, other reducing agents such as urea are also useful.

[0043]To control the operation of the reducing agent injector, the
reducing agent control system performs certain decision-making routines
about various operating characteristics of the reaction of reduction. The
gas analyzers provide information to the control system about operating
characteristics such as the amount and rate of injection of the reducing
agent, about the presence of the reducing agent in the gas mixture before
the reaction occurs, and about the success of the reaction in terms of
the extent of presence of nitrogen oxide and/or reducing agent in the gas
mixture after the reaction is completed. The reducing agent control
system controls the injection of reducing agent by calculating an initial
amount of reducing agent needed in view of the amount of nitrogen oxide
determined to be present in the gas mixture, and by adjusting that
calculation depending on the extent to which the reducing agent is
successfully incorporated into the gas mixture before the reaction
occurs, and depending on the extent to which nitrogen oxide has been
reacted out of the gas mixture without reducing agent slip.

[0044]The decision-making routine in the reducing agent control system is
run by a microprocessor chip, and applies one or more algorithms and/or
mathematical operations to that information to obtain a decision in the
form of a value that is equivalent to a desired state or condition that
should be possessed by a particular operating characteristic. Based on
the result of a decision-making routine, instructions are given by the
reducing agent control system that cause a change in the rate or amount
of injection of reducing agent thus moving the reduction reaction as
close as possible to ideal performance, which is characterized by minimal
residual nitrogen oxide and minimal reducing agent slip. In a preferred
embodiment of this invention, a gas mixture that contains a nitrogen
oxide that is reduced is, after the reduction reaction, free or
substantially free of nitrogen oxide, and/or is free or substantially
free of reducing agent.

[0045]In performing a decision-making routine, the reducing agent control
system may, and preferably does, employ a map. A map resides in a
read-only memory, and is an electronic collection of information about
various operating characteristics of the reaction of reduction. In one
embodiment, a range of quantified values may be set forth within the map
with respect to a particular operating characteristic. This could be, for
example, a range of temperature between 350 and 750° C., divided
into 25° C. increments. With respect to each individual value of
the parameter or operating characteristic in the range set forth, the map
may then associate an acceptable value for one or more other operating
characteristics, or a factor to be used in a decision-making routine. A
map can be established in the form of a relational database, and can be
accessed by look-up instructions in a computer program.

[0046]In the performance of a decision-making routine to control the
operation of the reaction of reducing a nitrogen oxide, a value, such as
the size of an electrical signal, that is representative of the state or
condition of operating characteristic A may be inputted to the reducing
agent control system. In one example of how the signal can then be
utilized by a decision-making routine, the microprocessor chip determines
a value representative of the state or condition each of operating
characteristics B and C, and reads the map to determine, in view of the
values for B and C, a target value D for operating characteristic A. The
target value could be a preselected value that is recorded in the map as
such, or could be a value that is calculated by the reducing agent
control system by a mathematical operation recorded in the map, with the
calculation to specify D being made only on the occasion when the values
for B and C are determined. For example, a determination may be made of
the absolute value of the difference between A and B, and this absolute
value, when added to C, becomes the target value D.

[0047]The value of operating characteristic A is compared to target value
D, and if A is in a desired relationship to D, the reducing agent control
system does not instruct that any adjustment in operations be made. If A
is not in a desired relationship to D, the decision-making process could,
in further alternative embodiments, read the map to determine a desired
value or range of values for A in terms of values for operating
characteristics E and F; or calculate a desired value for A by reading
the map to determine coefficients to be used in performing a mathematical
operation on E and F. The values for E and F could be determined at the
time of making the decision, or could be preselected values stored in the
map. In either case, once the desired value for A is determined, the
reducing agent control system instructs the necessary operating
characteristics of the reaction of reduction to be adjusted in the manner
necessary to obtain the desired value for A. This may be done by
adjusting operating characteristic A itself, or adjusting other operating
characteristics that can influence the state or condition of A. For
example, the reaction of reduction may be controlled by adjusting the
amount or frequency of injection of reducing agent, by adjusting the
timing of injection by injectors in different locations, by heating or
cooling the gas mixture or a reduction catalyst, and/or by adjusting the
operation of the emissions source such as by adjusting the fuel to air
ratio in a combustion reaction.

[0048]In this invention, information about the compositional content of
the gas emitted by a chemical reaction, such as the exhaust gas of a
source of combustion, may be used as an input to a decision-making in the
reducing agent control system. In the example described above,
information about combustion exhaust gas could be used as the
representative value that is inputted with respect to any one or more of
operating characteristics A, B, C, E or F, or could be used as a
coefficient in a operation that the decision-making routine causes to be
performed. Information about the gas composition is inputted to the
decision-making routine, in this invention, in the form of one or more
signals that is or are related to the individual concentration within the
emitted gas stream of a particular individual component gas therein, or a
particular subgroup of some but not all of the component gases therein,
or both an individual component and a subgroup. The relationship may be a
mathematical relationship, such as a monotonic relationship, involving
for example a log, inverse or scaled value. This is accomplished by
exposing a gas analyzer, such as an array of chemo/electro-active
materials, to the emitted gas stream to generate that may be, for
example, an electrical or optical signal.

[0049]The ability to furnish information about the individual
concentration within an emitted gas stream of a particular component gas
or subgroup therein makes it possible to calibrate a map. When building a
map before a reaction or device to be controlled is put into service,
values representative of a variety of parameters or operating
characteristics must be determined by systematically operating the
reaction or device under a large enough sample of different conditions to
approximate all the conditions expected in actual service. A gas
analyzer, such as an array of chemo/electro-active materials, can be used
to analyze the composition of the emitted gas stream to furnish
information based on the concentration of individual components or
subgroups therein to be recorded in the map in relation to values of
other parameters or operating characteristics measured under the same
operating conditions.

[0050]If preferred, however, this ability to furnish information related
to the concentration of individual components or subgroups in an emitted
gas stream can be used to calibrate or re-calibrate a map in real time
while the reduction reaction is in service. For example, a relationship
could be established in a map between a value representative of the
concentration of an individual gas component or subgroup, and values
representative of various parameters or operating characteristics, with
the value for the gas concentration to be supplied in real time. This
might take the form of a decision-making routine involving a mathematical
operation in which a value representative of the concentration of an
individual gas component or subgroup is used as a factor or coefficient.
The value representative of the concentration of an individual gas
component or subgroup could remain undetermined until the time that the
mathematical operation is performed in the execution of the
decision-making routine to make the decision. The value representative of
the concentration of an individual gas component or subgroup is
determined and supplied to the decision-making routine only on the
occasion of making the decision, and the decision thus need not be made
based on information that may not be currently accurate at the time the
decision is made. A map in which one or more parameters or operating
characteristics is related to information about the concentration of an
individual gas component or subgroup, with the information about the gas
concentration being furnished in real time while a reaction or device is
in service, clearly then has substantial value because it is possible to
essentially recalibrate the map continually in real time.

[0051]In this invention, information about an emitted gas composition may
be supplied to a map from a a gas analyzer employing one or more
chemo/electro-active materials that furnishes an analysis of the emitted
gas stream. Responses generated by the gas analyzer are then used as
inputs, optionally along with the input from other sensors such as a
temperature sensor, in the operation of algorithms that control the
reaction of reduction.

[0052]In the case again of an engine, there are several ways in which a
gas analyzer, such as an apparatus containing one or more
chemo/electro-active materials, can be incorporated into the operation of
a reducing agent control system to control the injection of reducing
agent and to control, ultimately, the reaction of reduction. The
chemo/electro-active materials may be constructed as an array of sensors
that have sensitivity to individual gaseous components or subgroups of
gases in a multi-component gas mixture, such as a stream of exhaust. Such
sensors can be fabricated from semiconducting materials that respond
uniquely to individual gases or gas subgroups that have common
characteristics such as similar oxidation potential, electronegativity,
or ability to form free radicals. These are properties of interest when
characterizing combustion.

[0053]Typical examples of individual gases and subgroups of gases within
an exhaust stream from a combustion reaction include oxygen, carbon
monoxide, hydrogen, sulfur dioxide, ammonia, CO2, H2S,
methanol, water, a hydrocarbon (such as CnH2n+2, and as same
may be saturated or unsaturated, or be optionally substituted with hetero
atoms; and cyclic and aromatic analogs thereof), a nitrogen oxide (such
as NO, NO2, N2O or N2O4) or an oxygenated carbon (CO,
CO2 or C5O3). The responses of an array of
chemo/electro-active materials to the multi-component mixture of such
gases formed by a stream of exhaust can thus be used to determine what
type of control over a reaction of reduction is needed to execute a
reaction in which nitrogen oxide content is decreased to the greatest
extent possible without engendering unacceptable reducing agent slip.

[0054]As an example, FIGS. 8 and 9 show several possible locations of a
gas analyzer, such as an array of sensor materials, in the exhaust system
of a vehicular internal combustion engine. The engine in FIGS. 8 and 9
contains a mass airflow and outside temperature sensor 60, an idle air
valve 62, a throttle position valve 64, an exhaust gas recycle valve 66,
an air temperature sensor 68, a pressure sensor 70, an air intake 72, an
intake manifold 74, fuel injectors 76, spark plugs 78, a crank position
sensor 80, a cam position sensor 82, a coolant temperature sensor 84, a
pre-catalytic converter 86, an emissions control device (such as a
catalytic converter and/or a device for the storage or abatement of NOx)
90, and a temperature sensor 92. The temperature sensor shown in FIGS. 8
and 9 need not be located adjacent the emissions control device 90 or the
SCR catalyst 104, or additional temperature sensors may be located
elsewhere along the exhaust conduit. FIG. 8 shows three possible
locations 94, 96, 98 for a gas analyzer, which may be upstream or
downstream from the emissions control device. The arrows indicate the
locations where it would be possible, if desired, to provide for the flow
of information to/from an engine control unit to/from one or more sensors
or actuators.

[0055]A gas analyzer at position 94 is located close to engine and
responds directly to the exhaust from individual cylinders. Because of
its proximity and fast response, the array in this location can be used
to obtain information from, or to control the operation of, each
individual cylinder. An array in this location is exposed to very high
exhaust temperatures for which semiconducting sensor materials are very
suitable. A gas sensor in position 96 in FIG. 8 operates cooler and is
exposed to gases that have already been modified in composition by the
precatalyst. However, the gas stream at this point still contains much
chemical information that can be used for control the reduction of
nitrogen oxides. This is also a suitable location to employ feed-forward
control by using an array of sensor materials to control operation of the
catalytic converter, which catalyzes the completion of the oxidation of
unburned fuel. Position 98 is a location that can be used to monitor
engine emissions and the current state of the catalytic converter. Based
on information from gas analyzer at this location, the catalytic
converter can be regenerated or otherwise controlled through feedback
process control.

[0056]FIG. 9 shows an SCR catalyst 104 and the deployment of gas sensors
in a control system in which a reducing agent is injected into the
exhaust conduit at position 110. Reducing agent is supplied from a
reservoir 102 and is passed through reducing agent control system 100 for
injection into the exhaust conduit. Reducing agent control system 100
includes the necessary pump to inject the reducing into the exhaust
conduit, and is connected to the microprocessor chip for the passage of
signals to and from the microprocessor chip to control the injection of
reducing agent. A gas analyzer, such as a gas sensor, can in this
arrangement be used either for feed-forward (position 106) or feedback
(position 108) control. The gas sensor is responsive to a variety of
gases that may be present in a combustion exhaust stream such as ammonia,
nitrogen oxide, carbon monoxide, oxygen, hydrocarbons and water. The
reducing agent control system, and the injection of reducing agent, may
be controlled by information obtained from a gas analyzer that is
positioned both upstream and/or downstream from a reduction catalyst and,
optionally, upstream and/or downstream from the reducing agent injector.
Information about the compositional content of the gas mixture containing
a nitrogen oxide is provided to a decision-making routine and/or map in
the microprocessor chip for processing into signals routed to the
reducing agent pump, to the engine itself or to heating or cooling
devices for the purpose of controlling the reaction of reduction.

[0057]An internal combustion engine, in which nitrogen oxide reduction is
controlled by the methods and apparatus of this invention, can be used
for many different purposes including, for example, in any type of
vehicle for transportation or recreation such as a car, truck, bus,
locomotive, aircraft, spacecraft, boat, jet ski, all-terrain vehicle or
snowmobile; or in equipment for construction, maintenance or industrial
operations such as pumps, lifts, hoists, cranes, generators, or equipment
for demolition, earth moving, digging, drilling, mining or
groundskeeping.

[0058]Although this invention has been described in detail with respect to
the control of the reduction of nitrogen oxides generated by combustion,
i.e. the oxidation of a fossil fuel, it is equally applicable to the
reduction of nitrogen oxides that may be found in a gas mixture generated
by any other type of chemical reaction. It is also equally applicable to
the reduction of nitrogen oxides that are not in a mixture with other
gases, where, for example, a gas analyzer is used to determine
information related to the relative concentration within the group of
nitrogen oxide of each individual nitrogen oxide. It is also equally
applicable to reducing agents in addition to ammonia and urea.

[0059]It will thus be seen that, in various embodiments of this invention,
as there may a plurality of reducing agent injectors, one or more gas
analyzers may be located in the conduit upstream or downstream from each
reducing agent injector. If a dust filter is used, it may be located
upstream from a reducing agent injector and/or one or more gas analyzers.

[0060]If a catalyst is present, the catalyst may be located upstream or
downstream from one or more gas analyzers. A first catalyst may be
located upstream from one or more gas analyzers, and a second catalyst
may be located downstream from one or more gas analyzers, particularly
where the catalyst is a plurality of vertically arranged catalyst beds. A
first gas analyzer may be located upstream from a catalyst, and a second
gas analyzer may be located downstream from the catalyst. One or more gas
analyzers may be located between first and second catalysts. One or more
gas analyzers may be located at the point of destination of a flowing
stream of a gas mixture, such as at a point of discharge to the
atmosphere.

[0061]If a gas analyzer outputs a signal to a decision-making routine, a
gas analyzer that is upstream from all catalyst, a gas analyzer that is
downstream from a first catalyst and upstream from a second catalyst,
and/or a gas analyzer that is downstream from all catalyst may each
output a signal to a decision-making routine. A gas analyzer may output
at least one signal that is related to the individual concentration
within the gas mixture of an individual nitrogen oxide component therein,
and/or may output at least one signal that is related to the collective
concentration within the gas mixture of all nitrogen oxide components
therein. The gas analyzer may in turn output a signal to a map. The gas
analyzer may also output a signal to a decision-making routine that
controls the injection of reducing agent, such as by calculating an
amount of reducing agent to be injected.

[0062]Information as to the compositional content of a gas mixture may be
determined before the injection of reducing agent, and/or before the gas
mixture contacts any catalyst. Information as to the compositional
content of a gas mixture may also be determined after the gas mixture
contacts a first catalyst but before the gas mixture contacts a second
catalyst, or after the gas mixture has contacted all catalyst. For
example, a gas analyzer that is upstream from all catalyst, and a gas
analyzer that is downstream from all catalyst may each output separate
signals to a decision-making routine.

[0063]The injection of the reducing agent may be controlled in relation to
such information as to the compositional content of the gas mixture, such
as by determining the amount of reducing agent to be injected into the
gas mixture. The information as to the compositional content of the gas
mixture may be an output of one or more gas analyzers, and may be related
to the individual concentration within the gas mixture of an individual
gas component therein (such as a nitrogen oxide), and/or related to the
collective concentration within the gas mixture of a subgroup of the
component gases therein (such as all nitrogen oxides).

[0064]In the present invention, an array of chemo/electro-active materials
is used for directly sensing one or more analyte gases in a
multi-component gas system under variable temperature conditions. By
"directly sensing" is meant that an array of gas-sensing materials will
be exposed to a mixture of gases that constitutes a multi-component gas
system, such as in a stream of flowing gases. The array may be situated
within the gas mixture, and more particularly within the source of the
gas mixture, if desired. Alternatively, although not preferred, the array
may reside in a chamber to which the gas mixture is directed from its
source at another location. When gas is directed to a chamber in which an
array is located, the gas mixture may be inserted in and removed from the
chamber by piping, conduits or any other suitable gas transmission
equipment.

[0065]A response may be obtained upon exposure of the gas-sensing
materials to the multi-component gas mixture, and the response will be a
function of the concentrations of one or more of the analyte gases
themselves in the gas mixture. The sensor materials will be exposed
simultaneously (or substantially simultaneously) to each of the analyte
gases, and an analyte gas does not have to be physically separated from
the multi-component gas mixture to be able to conduct an analysis of the
mixture and/or one or more analyte components thereof. This invention can
be used, for example, to obtain responses to, and thus to detect and/or
measure the concentrations, of combustion gases, such as oxygen, carbon
monoxide, nitrogen oxides, hydrocarbons such as butane, CO2,
H2S, sulfur dioxide, halogens, hydrogen, water vapor, an
organo-phosphorus gas, and ammonia, at variable temperatures in gas
mixtures such as automobile emissions.

[0066]This invention utilizes an array of sensing materials to analyze a
gas mixture and/or the components thereof to, for example, obtain a
response to, detect the presence of and/or calculate the concentration of
one or more individual analyte gas components in the system. By "array"
is meant at least two different materials that are spatially separated,
as shown for example in FIG. 1. The array may contain, for example, 3, 4,
5, 6, 8, 10 or 12 gas-sensing materials, or other greater or lesser
numbers as desired. It is preferred that there be provided at least one
sensor material for each of the individual gases or subgroups of gases in
the mixture to be analyzed. It may be desirable, however, to provide more
than one sensor material that is responsive to an individual gas
component and/or a particular subgroup of gases in the mixture. For
example, a group of at least 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12 sensors
could be used to detect the presence of, and/or calculate the
concentration of, one or more individual component gases and/or one or
more subgroups of gases in the mixture. Groups of sensors, which may or
may not have members in common, could be used to obtain a response to an
analyte that is an individual gas component or a subgroup of gases in the
mixture. A subgroup of gases that is, as the subgroup, an analyte may or
may not contain as a member an individual gas that is itself also an
analyte.

[0067]This invention is useful for detecting those gases that are expected
to be present in a gas stream. For example, in a combustion process,
gases that are expected to be present include oxygen, nitrogen oxides
(such as NO, NO2, N2O or N2O4), carbon monoxide,
hydrocarbons (such as CnH2n+2, and as same may be saturated or
unsaturated, or be optionally substituted with hetero atoms; and cyclic
and aromatic analogs thereof), ammonia or hydrogen sulfide, sulfur
dioxide, CO2, or methanol. Other gases of interest may include
alcohol vapors, solvent vapors, hydrogen, water vapor, and those deriving
from saturated and unsaturated hydrocarbons, ethers, ketones, aldehydes,
carbonyls, biomolecules and microorganisms. The component of a
multi-component gas mixture that is an analyte of interest may be an
individual gas such as carbon monoxide; may be a subgroup of some but not
all of the gases contained in the mixture, such as the nitrogen oxides
(NOx) or hydrocarbons; or may be a combination of one or more
individual gases and one or more subgroups. When a subgroup of gases is
an analyte, a chemo/electro-active material will respond to the
collective concentration within a multi-component gas mixture of the
members of the subgroup together.

[0068]The analyte gas(es) contained in the mixture to which the
chemo/electro-active material will be exposed can be a single gas, a
subgroup of gases together, or one or more gases or subgroups mixed with
an inert gas such as nitrogen. Particular gases of interest are donor and
acceptor gases. These are gases that either donate electrons to the
semiconducting material, such as carbon monoxide, H2S and
hydrocarbons, or accept electrons from the semiconducting material, such
as O2, nitrogen oxides (commonly depicted as NOx), and
halogens. When exposed to a donor gas, an n-type semiconducting material
will have a decrease in electrical resistance, increasing the current,
and it, therefore, will show an increase in temperature due to I2R
heating. When exposed to an acceptor gas, an n-type semiconducting
material will have an increase in electrical resistance, decreasing the
current, and therefore will show a decrease in temperature due to
I2R heating. The opposite occurs in each instance with p-type
semiconducting materials.

[0069]Obtaining information related to the compositional content of a gas
mixture using these sensor materials, such as measurement of gas
concentrations, can be based on a change in an electrical property, such
as AC impedance, of at least one, but preferably each and all, of the
materials upon exposure of the materials to a mixture containing one or
more analyte gases. Analysis of a gas mixture can also be performed in
terms of extent of change in other electrical properties of the sensor
materials, such as capacitance, voltage, current or AC or DC resistance.
Change in DC resistance may be determined, for example, by measuring
change in temperature at constant voltage. The change in one of these
illustrative properties of a sensor material is a function of the partial
pressure of an analyte gas within the gas mixture, which in turn
determines the concentration at which the molecules of the analyte gases
become adsorbed on the surface of a sensor material, thus affecting the
electrical response characteristics of that material. By using an array
of chemo/electro-active materials, a pattern of the respective responses
exhibited by the materials upon exposure to a mixture containing one or
more analyte gases can be used to simultaneously and directly detect the
presence of, and/or measure the concentration of, at least one gas in a
multi-component gas system. The invention, in turn, can be used to
determine the composition of the gas system. The concept is illustrated
schematically in FIG. 1 and is exemplified below.

[0070]To illustrate, consider the theoretical example below of the
exposure of a sensor material to a mixture containing an analyte gas.
Where a response is obtained, the event is depicted as positive (+), and
where no response is obtained, the event is depicted as negative (-).
Material 1 responds to Gas 1 and Gas 2, but shows no response to Gas 3.
Material 2 responds to Gas 1 and Gas 3, but shows no response to Gas 2,
and Material 3 responds to Gas 2 and Gas 3, but shows no response to Gas
1.

[0071]Therefore, if an array consisting of Materials 1, 2 and 3 gives the
following response to an unknown gas,

TABLE-US-00002
Material 1 Material 2 Material 3
Unknown Gas + - +

then the unknown gas would be identified as Gas 2. The response of each
sensor material would be a function of the partial pressure within the
mixture of, and thus the concentration of, an analyte gas or the
collective concentration of a subgroup of analyte gases; and the response
could be quantified or recorded as a processible value, such as a
numerical value. In such case, the values of one or more responses can be
used to generate quantitative information about the presence within the
mixture of one or more analyte gases. In a multicomponent gas system,
chemometrics, neural networks or other pattern recognition techniques
could be used to calculate the concentration of one or more analyte gases
in the mixture of the system.

[0072]The sensing materials used are chemo/electro-active materials. A
"chemo/electro-active material" is a material that has an electrical
response to at least one individual gas in a mixture. Some metal oxide
semiconducting materials, mixtures thereof, or mixtures of metal oxide
semiconductors with other inorganic compounds are chemo/electro-active,
and are particularly useful in this invention. Each of the various
chemo/electro-active materials used herein preferably exhibits an
electrically detectable response of a different kind and/or extent, upon
exposure to the mixture and/or an analyte gas, than each of the other
chemo/electro-active materials. As a result, an array of appropriately
chosen chemo/electro-active materials can be used to analyze a
multi-component gas mixture, such as by interacting with an analyte gas,
sensing an analyte gas, or determining the presence and/or concentration
of one or more analyte gases or subgroups in a mixture, despite the
presence therein of interfering gases that are not of interest.
Preferably the mole percentages of the major components of each
gas-sensing material differs from that of each of the others.

[0073]The chemo/electro-active material can be of any type, but especially
useful are semiconducting metal oxides such as SnO2, TiO2,
WO3 and ZnO. These particular materials are advantageous due to
their chemical and thermal stability. The chemo/electro-active material
can be a mixture of two or more semiconducting materials, or a mixture of
a semiconducting material with an inorganic material, or combinations
thereof. The semiconducting materials of interest can be deposited on a
suitable solid substrate that is an insulator such as, but not limited
to, alumina or silica and is stable under the conditions of the
multi-component gas mixture. The array then takes the form of the sensor
materials as deposited on the substrate. Other suitable sensor materials
include single crystal or polycrystalline semiconductors of the bulk or
thin film type, amorphous semiconducting materials, and semiconductor
materials that are not composed of metal oxides.

[0074]In various embodiments, the substrate may be a high-temperature
multilayer ceramic, which is prepared from Al2O3, AlN, and, to
a smaller extent, BeO and SiC. The alumina content is dominant, however,
with about 92-96 weight % of the composition being Al2O3. The
structure consists of many layers of ceramic, with metallization between
the layers, and via holes through the layers for electrical contact. A
well known application of large modules with many layers of ceramic is
IBM's pioneering product "Thermal Conduction Module" (TCM) for mainframe
computers in 1983. The module had 33 layers, and 133 silicon chips were
mounted by flip chip soldering.

[0075]This type of non-sintered, pliable ceramic consists of alumina
powder, organic binders and solvents. The material is spread from a
container down on a transport carrier underneath. The ceramic "tape"
("green sheet") is given the appropriate thickness on the transport
carrier by passing underneath a "doctor blade" in a precisely controlled
distance. The tape is cut to correct size, and holes and component
cavities are punched out with a numerically controlled punching tool, or
with a permanent, product specific punching tool for high production
volume of a given product. Metallization of the via holes and fabrication
of conductors is done by screen printing of tungsten (or molybdenum).
These are the only metals that can withstand the high process temperature
during the subsequent sintering process. All layers are laminated
together under hydrostatic (or uni-axial) pressure at elevated
temperature (500-600° C.) to evaporate the binder and solvent.
Then the whole structure is sintered at 1370-1650° C., 30-50
hours, in a hydrogen atmosphere.

[0076]For small circuits, many modules are made on one substrate, and the
individual circuits can be parted by breaking the substrate at the end of
the process. Then the external contacts are brazed to the substrate, and
finally gold may be plated on the surface with nickel as a diffusion
barrier on top of the tungsten. The plating is preferably done
electrolytically to achieve sufficient thickness and good conductivity if
an electrical contact to all parts of the conductor pattern can be made.
Otherwise, chemical plating is used.

[0077]During the process, the ceramic shrinks approximately 18% linearly.
This is taken into consideration during the design of the circuit, both
sideways and in thickness, which affects the characteristic impedance. As
the shrinkage is material and process dependent, the finished circuits
typically have linear dimensional tolerances 0.5-1%. These ceramic
substrates have low TCE, a good thermal match to Si and GaAs as well as
to leadless SMD components, good control over characteristic impedance,
and good high frequency properties. Many layers are possible with high
production yield because each layer can be inspected before the
lamination, and faulty layers can be discarded. Among the disadvantages
are low electrical conductivity in the inner layers (sheet
resistivity˜15 mohm/sq), and high dielectric constant, which gives
delay, inferior pulse rise time and increased power loss and cross talk
at very high frequencies.

[0078]The chemo/electro-active materials that contain more than one metal
do not have to be a compound or solid solution, but can be a multi-phase
physical mixture of discrete metals and/or metal oxides. As there will be
varying degrees of solid state diffusion by the precursor materials from
which the chemo/electro-active materials are formed, the final materials
may exhibit composition gradients, and they can be crystalline or
amorphous. Suitable metal oxides are those that [0079]i) when at a
temperature of about 400° C. or above, have a resistivity of about
1 to about 106 ohm-cm, preferably about 1 to about 105 ohm-cm,
and more preferably about 10 to about 104 ohm-cm; [0080]ii) show a
chemo/electro response to at least one gas of interest; and [0081]iii)
are stable and have mechanical integrity, that is are able to adhere to
the substrate and not degrade at the operating temperature.The metal
oxides may also contain minor or trace amounts of hydration and elements
present in the precursor materials.

[0082]The sensor materials may optionally contain one or more additives to
promote adhesion to a substrate, or that alter the conductance,
resistance or selectivity of the sensor material. Examples of additives
to alter the conductance, resistance or selectivity of the sensor
material include Ag, Au or Pt, as well as frits. Examples of additives to
promote adhesion include frits, which are finely ground inorganic
minerals that are transformed into glass or enamel on heating, or a
rapidly quenched glass that retains its amorphous quality in the solid
state. Frit percursor compounds are melted at high temperature and
quenched, usually by rapidly pouring the melt into a fluid such as water,
or by pouring through spinning metal rollers. The precursor compounds
usually are a mechanical mixture of solid compounds such as oxides,
nitrates or carbonates, or can be co-precipitated or gelled from a
solution. Suitable precursor materials for frits include alkali and
alkaline earth alumino-silicates and alumino-boro-silicates, copper,
lead, phosphorus, titanium, zinc and zirconium. Frits as additives may be
used in amounts of up to 30 volume percent, and preferably up to 10
volume percent, of the total volume of the chemo/electro-active material
from which the sensor is made.

[0083]If desired, the sensor materials may also contain additives that,
for example, catalyze the oxidation of a gas of interest or promote the
selectivity for a particular analyte gas; or contain one or more dopants
that convert an n semiconductor to a p semiconductor, or vice versa.
These additives may be used in amounts of up to 30 weight percent, and
preferably up to 10 weight percent, of the chemo/electro-active material
from which the sensor is made.

[0084]Any frits or other additives used need not be uniformly or
homogeneously distributed throughout the sensor material as fabricated,
but may be localized on or near a particular surface thereof as desired.
Each chemo/electro-active material may, if desired, be covered with a
porous dielectric overlayer.

[0085]The chemo/electro-active materials used as sensor materials in this
invention may, for example, be metal oxides of the formula
M1Ox, M1aM2bOx, or
M1aM2bM3cOx; or mixtures thereof,
wherein [0086]M1, M2 and M3 are metals that form stable
oxides when fired in the presence of oxygen above 500° C.;
[0087]M1 is selected from Periodic Groups 2-15 and the lanthanide
group; [0088]M2 and M3 are each independently selected from
Periodic Groups 1-15 and the lanthanide group; [0089]M1 and M2
are not the same in M1aM2bOx, and M1,
M2 and M3 are not the same in
M1aM2bM3cOx; [0090]a, b, and c are
each independently in the range of about 0.0005 to about 1; and [0091]x
is a number sufficient so that the oxygen present balances the charges of
the other elements present in the chemo/electro-active material.

[0092]In certain preferred embodiments, the metal oxide materials may
include those in which [0093]M1 is selected from the group
consisting of Ce, Co, Cu, Fe, Ga, Nb, Ni, Pr, Ru, Sn, Ti, Tm, W, Yb, Zn,
and Zr; and/or [0094]M2 and M3 are each independently selected
from the group consisting of Al, Ba, Bi, Ca, Cd, Ce, Co, Cr, Cu, Fe, Ga,
Ge, In, K, La, Mg, Mn, Mo, Na, Nb, Ni, Pb, Pr, Rb, Ru, Sb, Sc, Si, Sn,
Sr, Ta, Ti, Tm, V, W, Y, Yb, Zn, and Zr;but in which M1 and M2
are not the same in M1aM2bOx, and M1,
M2 and M3 are not the same in
M1aM2bM3cOx.

[0095]In certain other preferred embodiments, the metal oxide materials
may include those in which

[0099]In certain other preferred embodiments, the metal oxide materials
may include those that are in an array of first and second
chemo/electro-active materials, wherein the chemo/electro-active
materials are selected from the pairings in the group consisting of
[0100](i) the first material is M1Ox, and the second material
is M1aM2bOx; [0101](ii) the first material is
M1Ox, and the second material is
M1aM2bM3cOx; [0102](iii) the first
material is M1aM2bOx, and the second material is
M1aM2bM3cOx; [0103](iv) the first
material is a first M1Ox; and the second material is a second
M1Ox; [0104](v) the first material is a first
M1aM2bOx, and the second material is a second
M1aM2bOx; and [0105](vi) the first material is a
first M1aM2bM3cOx, and the second
material is a second
M1aM2bM3cOx;wherein

[0108]but M1 and M2 are not the same in
M1aM2bOx, and M1, M2 and M3 are
not the same in M1aM2bM3cOx;

[0109]a, b and c are each independently about 0.0005 to about 1; and

[0110]x is a number sufficient so that the oxygen present balances the
charges of the other elements present in the chemo/electro-active
material.

[0111]In certain other preferred embodiments, an array of two or more
chemo/electro-active materials may be selected from the group consisting
of (i) the chemo/electro-active materials that include M1Ox,
(ii) the chemo/electro-active materials that include
M1aM2bOx, and (iii) the chemo/electro-active
materials that include M1aM2bM3cOx;
[0112]wherein M1 is selected from the group consisting of Al, Ce,
Cr, Cu, Fe, Ga, Mn, Nb, Ni, Pr, Sb, Sn, Ta, Ti, W and Zn; [0113]wherein
M2 and M3 are each independently selected from the group
consisting of Ga, La, Mn, Ni, Sn, Sr, Ti, W, Y, Zn; [0114]wherein M1
and M2 are each different in M1aM2bOx, and
M1, M2 and M3 are each different in
M1aM2bM3cOx; [0115]wherein a, b and c
are each independently about 0.0005 to about 1; and [0116]wherein x is a
number sufficient so that the oxygen present balances the charges of the
other elements in the chemo/electro-active material.

[0117]M1 may for example be selected from the group consisting of Al,
Cr, Fe, Ga, Mn, Nb, Ni, Sb, Sn, Ta, Ti and Zn, or from the group
consisting of Ga, Nb, Ni, Sb, Sn, Ta, Ti and Zn. M2, M3, or
M2 and M3 may be selected from the group consisting of La, Ni,
Sn, Ti and Zn, or the group consisting of Sn, Ti and Zn.

[0118]The array may contain other numbers of chemo/electro-active
materials such as four or eight, and the array may contain at least one
chemo/electro-active material that comprises M1Ox, and at least
three chemo/electro-active materials that each comprise
M1aM2bOx. Alternatively, the array may contain
(i) at least one chemo/electro-active material that comprises
M1Ox, and at least four chemo/electro-active materials that
each comprise M1aM2bOx; or (ii) at least two
chemo/electro-active materials that each comprise M1Ox, and at
least four chemo/electro-active materials that each comprise
M1aM2bOx; or (iii) at least three
chemo/electro-active materials that each comprise
M1aM2bOx, and at least one chemo/electro-active
material that comprises M1aM2bM3cOx.

[0119]Chemo/electro-active materials useful in the apparatus of this
invention may be selected from one or more members of the group
consisting of [0120]a chemo/electro-active material that comprises
AlaNibOx[0121]a chemo/electro-active material that
comprises CeO2, [0122]a chemo/electro-active material that comprises
CraMnbOx, [0123]a chemo/electro-active material that
comprises CraTibOx[0124]a chemo/electro-active material
that comprises CraYbOx[0125]a chemo/electro-active
material that comprises CuaGabOx, [0126]a
chemo/electro-active material that comprises CuaLabOx[0127]a chemo/electro-active material that comprises CuO, [0128]a
chemo/electro-active material that comprises FeaLabOx[0129]a chemo/electro-active material that comprises
FeaNibOx[0130]a chemo/electro-active material that
comprises FeaTibOx[0131]a chemo/electro-active material
that comprises GaaTibZncOx[0132]a
chemo/electro-active material that comprises MnaTibOx[0133]a chemo/electro-active material that comprises
NbaSrbOx, [0134]a chemo/electro-active material that
comprises NbaTibOx[0135]a chemo/electro-active material
that comprises NbaTibZncOx[0136]a
chemo/electro-active material that comprises NbaWbOx[0137]a chemo/electro-active material that comprises NiO, [0138]a
chemo/electro-active material that comprises NiaZnbOx[0139]a chemo/electro-active material that comprises Pr6O11,
[0140]a chemo/electro-active material that comprises
SbaSnbOx. [0141]a chemo/electro-active material that
comprises SnO2, [0142]a chemo/electro-active material that comprises
TaaTibOx, and [0143]a chemo/electro-active material that
comprises TiaZnbOx. [0144]a chemo/electro-active material
that comprises WO3, and [0145]a chemo/electro-active material that
comprises ZnO.wherein a, b and c are each independently about 0.0005 to
about 1; and wherein x is a number sufficient so that the oxygen present
balances the charges of the other elements in the chemo/electro-active
material.

[0146]Chemo/electro-active materials useful in this invention may also be
selected from subgroups of the foregoing formed by omitting any one or
more members from the whole group as set forth in the list above. As a
result, the chemo/electro-active materials may in such instance not only
be any one or more member(s) selected from any subgroup of any size that
may be formed from the whole group as set forth in the list above, but
the subgroup may also exclude the members that have been omitted from the
whole group to form the subgroup. The subgroup formed by omitting various
members from the whole group in the list above may, moreover, contain any
number of the members of the whole group such that those members of the
whole group that are excluded to form the subgroup are absent from the
subgroup. Representative subgroups are set forth below.

[0155]Of the above, one or more members of the group consisting of
[0156]a chemo/electro-active material that comprises CeO2, [0157]a
chemo/electro-active material that comprises SnO2, and [0158]a
chemo/electro-active material that comprises ZnOmay contain a frit
additive.

[0194]In the apparatus of this invention, a chemo/electro-active material
that comprises M1aM2bOx may be selected from the
group consisting of [0195]a chemo/electro-active material that
comprises AlaNibOx[0196]a chemo/electro-active material
that comprises CraTibOx, and [0197]a chemo/electro-active
material that comprises FeaLabOx.or the group consisting
of [0198]a chemo/electro-active material that comprises
CraTibOx[0199]a chemo/electro-active material that
comprises FeaLabOx, and [0200]a chemo/electro-active
material that comprises FeaNibOx or the group consisting
of [0201]a chemo/electro-active material that comprises
FeaLabOx[0202]a chemo/electro-active material that
comprises FeaNibOx, and [0203]a chemo/electro-active
material that comprises NiaZnbOx or the group consisting
of [0204]a chemo/electro-active material that comprises
FeaNibOx[0205]a chemo/electro-active material that
comprises NiaZnbOx, and [0206]a chemo/electro-active
material that comprises SbaSnbOx.or the group consisting
of [0207]a chemo/electro-active material that comprises
AlaNibOx[0208]a chemo/electro-active material that
comprises CraTibOx[0209]a chemo/electro-active material
that comprises FeaLabOx[0210]a chemo/electro-active
material that comprises FeaNibOx[0211]a
chemo/electro-active material that comprises NiaZnbOx, and
[0212]a chemo/electro-active material that comprises
SbaSnbOx.or the group consisting of [0213]a
chemo/electro-active material that comprises AlaNibOx[0214]a chemo/electro-active material that comprises
CraTibOx, and [0215]a chemo/electro-active material that
comprises MnaTibOx or the group consisting of [0216]a
chemo/electro-active material that comprises NbaTibOx[0217]a chemo/electro-active material that comprises
NiaZnbOx, and [0218]a chemo/electro-active material that
comprises SbaSnbOx or the group consisting of [0219]a
chemo/electro-active material that comprises NiaZnbOx[0220]a chemo/electro-active material that comprises
SbaSnbOx, and [0221]a chemo/electro-active material that
comprises TaaTibOx or the group consisting of [0222]a
chemo/electro-active material that comprises SbaSnbOx[0223]a chemo/electro-active material that comprises
TaaTibOx, and [0224]a chemo/electro-active material that
comprises TiaZnbOx.or the group consisting of [0225]a
chemo/electro-active material that comprises CraMnbOx[0226]a chemo/electro-active material that comprises
CraTibOx, and [0227]a chemo/electro-active material that
comprises CraYbOx or the group consisting of [0228]a
chemo/electro-active material that comprises CraTibOx[0229]a chemo/electro-active material that comprises
CraYbOx, and [0230]a chemo/electro-active material that
comprises CuaGabOx or the group consisting of [0231]a
chemo/electro-active material that comprises CraYbOx[0232]a chemo/electro-active material that comprises
CuaGabOx, and [0233]a chemo/electro-active material that
comprises CuaLabOx or the group consisting of [0234]a
chemo/electro-active material that comprises CuaGabOx[0235]a chemo/electro-active material that comprises
CuaLabOx, and [0236]a chemo/electro-active material that
comprises FeaLabOx.or the group consisting of [0237]a
chemo/electro-active material that comprises CraMnbOx[0238]a chemo/electro-active material that comprises
CraTibOx[0239]a chemo/electro-active material that
comprises CraYbOx[0240]a chemo/electro-active material
that comprises CuaGabOx[0241]a chemo/electro-active
material that comprises CuaLabOx, and [0242]a
chemo/electro-active material that comprises FeaLabOx.or
the group consisting of [0243]a chemo/electro-active material that
comprises CraYbOx[0244]a chemo/electro-active material
that comprises CuaGabOx, and [0245]a chemo/electro-active
material that comprises CuaLabOx or the group consisting
of [0246]a chemo/electro-active material that comprises
CuaGabOx, [0247]a chemo/electro-active material that
comprises CuaLabOx, and [0248]a chemo/electro-active
material that comprises FeaTibOx or the group consisting
of [0249]a chemo/electro-active material that comprises
CraMnbOx[0250]a chemo/electro-active material that
comprises MnaTibOx, and [0251]a chemo/electro-active
material that comprises NbaSrbOx

[0252]In the apparatus of this invention, a chemo/electro-active material
that comprises M1aM2bOx, or a chemo/electro-active material that
comprises M1aM2bM3cOx, may be selected from the group consisting of
[0253]a chemo/electro-active material that comprises
CraTibOx[0254]a chemo/electro-active material that
comprises MnaTibOx, and [0255]a chemo/electro-active
material that comprises NbaTibZncOx or the group
consisting of [0256]a chemo/electro-active material that comprises
MnaTibOx[0257]a chemo/electro-active material that
comprises NbaTibZncOx, and [0258]a
chemo/electro-active material that comprises TaaTibOx or
the group consisting of [0259]a chemo/electro-active material that
comprises NbaTibZncOx[0260]a chemo/electro-active
material that comprises TaaTibOx, and [0261]a
chemo/electro-active material that comprises TiaZnbOx.or
the group consisting of [0262]a chemo/electro-active material that
comprises AlaNibOx[0263]a chemo/electro-active material
that comprises CraTibOx[0264]a chemo/electro-active
material that comprises MnaTibOx[0265]a
chemo/electro-active material that comprises
NbaTibZncOx[0266]a chemo/electro-active material
that comprises TaaTibOx, and [0267]a chemo/electro-active
material that comprises TiaZnbOx.or the group consisting
of [0268]a chemo/electro-active material that comprises
GaaTibZncOx[0269]a chemo/electro-active material
that comprises NbaTibOx, and [0270]a chemo/electro-active
material that comprises NiaZnbOx or the group consisting
of [0271]a chemo/electro-active material that comprises
GaaTibZncOx[0272]a chemo/electro-active material
that comprises NbaTibOx[0273]a chemo/electro-active
material that comprises NiaZnbOx[0274]a
chemo/electro-active material that comprises SbaSnbOx[0275]a chemo/electro-active material that comprises
TaaTibOx, and [0276]a chemo/electro-active material that
comprises TiaZnbOx.or the group consisting of [0277]a
chemo/electro-active material that comprises CuaLabOx[0278]a chemo/electro-active material that comprises
FeaTibOx, and [0279]a chemo/electro-active material that
comprises GaaTibZncOx or the group consisting of
[0280]a chemo/electro-active material that comprises
FeaTibOx[0281]a chemo/electro-active material that
comprises GaaTibZncOx, and [0282]a
chemo/electro-active material that comprises NbaWbOx.or
the group consisting of [0283]a chemo/electro-active material that
comprises CraYbOx[0284]a chemo/electro-active material
that comprises CuaGabOx, [0285]a chemo/electro-active
material that comprises CuaLabOx[0286]a
chemo/electro-active material that comprises FeaTibOx[0287]a chemo/electro-active material that comprises
GaaTibZncOx, and [0288]a chemo/electro-active
material that comprises NbaWbOx.or the group consisting of
[0289]a chemo/electro-active material that comprises
MnaTibOx[0290]a chemo/electro-active material that
comprises NbaSrbOx, and [0291]a chemo/electro-active
material that comprises NbaTibZncOx

[0292]In the apparatus of this invention, a chemo/electro-active material
that comprises M1Ox, a chemo/electro-active material that comprises
M1aM2bOx, or a chemo/electro-active material that comprises M1aM2bM3cOx,
may be selected from the group consisting of [0293]a
chemo/electro-active material that comprises
GaaTibZncOx[0294]a chemo/electro-active material
that comprises NbaTibOx[0295]a chemo/electro-active
material that comprises NiaZnbOx, and [0296]a
chemo/electro-active material that comprises SnO2 or the group
consisting of [0297]a chemo/electro-active material that comprises
GaaTibZncOx[0298]a chemo/electro-active material
that comprises NbaTibOx[0299]a chemo/electro-active
material that comprises NiaZnbOx[0300]a
chemo/electro-active material that comprises SnO2, [0301]a
chemo/electro-active material that comprises TaaTibOx, and
[0302]a chemo/electro-active material that comprises
TiaZnbOx.or the group consisting of [0303]a
chemo/electro-active material that comprises NbaSrbOx[0304]a chemo/electro-active material that comprises
NbaTibZncOx, and [0305]a chemo/electro-active
material that comprises Pr6O11 or the group consisting of
[0306]a chemo/electro-active material that comprises
NbaTibZncOx[0307]a chemo/electro-active material
that comprises Pr6O11, and [0308]a chemo/electro-active
material that comprises TiaZnbOx.or the group consisting
of [0309]a chemo/electro-active material that comprises
CraMnbOx[0310]a chemo/electro-active material that
comprises MnaTibOx[0311]a chemo/electro-active material
that comprises NbaSrbOx[0312]a chemo/electro-active
material that comprises NbaTibZncOx[0313]a
chemo/electro-active material that comprises Pr6O11, and
[0314]a chemo/electro-active material that comprises
TiaZnbOx.

[0315]In the apparatus of this invention, a chemo/electro-active material
that comprises M1Ox, or a chemo/electro-active material that comprises
M1aM2bOx may be selected from the group consisting of [0316]a
chemo/electro-active material that comprises NbaTibOx[0317]a chemo/electro-active material that comprises
NiaZnbOx, and [0318]a chemo/electro-active material that
comprises SnO2.or the group consisting of [0319]a
chemo/electro-active material that comprises NiaZnbOx[0320]a chemo/electro-active material that comprises SnO2, and
[0321]a chemo/electro-active material that comprises
TaaTibOx or the group consisting of [0322]a
chemo/electro-active material that comprises SnO2, [0323]a
chemo/electro-active material that comprises TaaTibOx, and
[0324]a chemo/electro-active material that comprises
TiaZnbOx.or the group consisting of [0325]a
chemo/electro-active material that comprises NbaTibOx[0326]a chemo/electro-active material that comprises
NiaZnbOx[0327]a chemo/electro-active material that
comprises SbaSnbOx, and [0328]a chemo/electro-active
material that comprises ZnO.or the group consisting of [0329]a
chemo/electro-active material that comprises NiaZnbOx[0330]a chemo/electro-active material that comprises
SbaSnbOx[0331]a chemo/electro-active material that
comprises TaaTibOx, and [0332]a chemo/electro-active
material that comprises ZnOor the group consisting of [0333]a
chemo/electro-active material that comprises SbaSnbOx[0334]a chemo/electro-active material that comprises
TaaTibOx[0335]a chemo/electro-active material that
comprises TiaZnbOx, and [0336]a chemo/electro-active
material that comprises ZnOor the group consisting of [0337]a
chemo/electro-active material that comprises TaaTibOx[0338]a chemo/electro-active material that comprises
TiaZnbOx, and [0339]a chemo/electro-active material that
comprises ZnO.or the group consisting of [0340]a chemo/electro-active
material that comprises NbaTibOx[0341]a
chemo/electro-active material that comprises NiaZnbOx[0342]a chemo/electro-active material that comprises
SbaSnbOx[0343]a chemo/electro-active material that
comprises TaaTibOx[0344]a chemo/electro-active material
that comprises TiaZnbOx, and [0345]a chemo/electro-active
material that comprises ZnO.or the group consisting of [0346]a
chemo/electro-active material that comprises AlaNibOx[0347]a chemo/electro-active material that comprises
CraMnbOx, and [0348]a chemo/electro-active material that
comprises CuOor the group consisting of [0349]a chemo/electro-active
material that comprises CraMnbOx[0350]a
chemo/electro-active material that comprises CuO, and [0351]a
chemo/electro-active material that comprises NbaSrbOx or
group consisting of [0352]a chemo/electro-active material that comprises
CuO [0353]a chemo/electro-active material that comprises
NbaSrbOx, and [0354]a chemo/electro-active material that
comprises Pr6O11 or group consisting of [0355]a
chemo/electro-active material that comprises NbaSrbOx[0356]a chemo/electro-active material that comprises Pr6O11,
and [0357]a chemo/electro-active material that comprises WO3.or
group consisting of [0358]a chemo/electro-active material that comprises
AlaNibOx[0359]a chemo/electro-active material that
comprises CraMnbOx[0360]a chemo/electro-active material
that comprises CuO [0361]a chemo/electro-active material that comprises
NbaSrbOx[0362]a chemo/electro-active material that
comprises Pr6O11, and [0363]a chemo/electro-active material
that comprises WO3.

[0364]Any method of depositing the chemo/electro-active material to a
substrate is suitable. One technique used for deposition is applying a
semiconducting material on an alumina substrate on which electrodes are
screen printed. The semiconducting material can be deposited on top of
electrodes by hand painting semiconducting materials onto the substrate,
pipetting materials into wells, thin film deposition, or thick film
printing techniques. Most techniques are followed by a final firing to
sinter the semiconducting materials.

[0365]Techniques for screen-printing substrates with the electrodes and
chemo/electro-active materials are illustrated in FIGS. 2-3. FIG. 2
depicts a method of using interdigitated electrodes overlaid with
dielectric material, forming blank wells into which the
chemo/electro-active materials can be deposited. FIG. 3 depicts an
electrode screen pattern for an array of 6 materials which is printed on
both sides of the substrate to provide for a 12-material array chip. Two
of the electrodes are in parallel so it holds only 6 unique materials.
Counting down from the top of the array shown in FIG. 3, the top two
materials can only be accessed simultaneously by the split electrode with
which they have shared contact. Below that is the screen pattern for the
dielectric material, which is screen printed on top of the electrodes on
both sides of the substrate to prevent the material from being fouled by
contact with the gas mixture, such as a deposit of soot that could reduce
the sensitivity of a sensor material to a gas or cause a short. Below
that is the screen pattern for the actual sensor materials. This is
printed in the holes in the dielectric on top of the electrodes. When
more than one material is used in the array, the individual materials are
printed one at a time.

[0366]The geometry of a sensor material as fabricated in an array,
including such characteristics as its thickness, selection of a compound
or composition for use as the sensor, and the voltage applied across the
array, can vary depending on the sensitivity required. If desired, the
apparatus may be constructed in a size such that it may be passed through
an opening that is the size of a circle having a diameter of no more than
about 150 mm, or no more than about 100 mm, or no more than about 50 mm,
or no more than about 25 mm, or no more than about 18 mm, as the
requirements of it usage may dictate. The sensor materials are preferably
connected in parallel in a circuit to which a voltage of about 1 to about
20, preferably about 1 to about 12, volts is applied across the sensor
materials.

[0367]As noted, the types of electrical response characteristics that may
be measured include AC impedance or resistance, capacitance, voltage,
current or DC resistance. It is preferred to use resistance as the
electric response characteristic of a sensor material that is measured to
perform analysis of a gas mixture and/or a component therein. For
example, a suitable sensor material may be that which, when at a
temperature of about 400° C. or above, has a resistivity of at
least about 1 ohm-cm, and preferably at least about 10 ohm-cm, and yet no
more than about 106 ohm-cm, preferably no more than about 105
ohm-cm, and more preferably no more than about 104 ohm-cm. Such a
sensor material may also be characterized as that which exhibits,
preferably at a temperature of about 400° C. or above, upon
exposure to a gas mixture, a change in resistance of at least about 0.1
percent, and preferably at least about 1 percent, as compared to the
resistance in the absence of exposure. Using such material, a signal may
be generated that is proportional to the resistance of exhibited by the
material when it is exposed to a multi-component gas mixture.

[0368]Regardless of the type of response characteristic that is measured
for the purpose of analyzing a mixture and/or a gaseous component of
interest therein, it is desirable that a sensor material be utilized for
which a quantified value of that response characteristic is stable over
an extended period of time. When the sensor material is exposed to a
mixture containing the analyte, the concentration of the analyte being a
function of the composition of the particular gas mixture in which it is
contained, the value of the response of the sensor material will
preferably remain constant or vary to only a small extent during exposure
to the mixture over an extended period of time at a constant temperature.
For example, the value of the response, if it varies, will vary by no
more than about twenty percent, preferably no more than about ten
percent, more preferably no more than about five percent, and most
preferably no more than about one percent over a period of at least about
1 minute, or preferably a period of hours such as at least about 1 hour,
preferably at least about 10 hours, more preferably at least about 100
hours, and most preferably at least about 1000 hours. One of the
advantages of the types of sensor materials described above is that they
are characterized by this kind of stability of response.

[0369]The electrical response characteristic exhibited by a
chemo/electro-active material in respect of a multi-component gas mixture
that contains an analyte gas or sub-group of gases derives from contact
of the surface of the chemo/electro-active material with the gas mixture
containing the analyte(s). The electrical response characteristic is an
electrical property, such as capacitance, voltage, current, AC impedance,
or AC or DC resistance, that is affected by exposure of the
chemo/electro-active material to the multi-component gas mixture. A
quantified value of, or a signal proportional to the quantified value of,
the electrical property or a change in the electrical property may be
obtained as a useful measurement at one or more times while the material
is exposed to the gas mixture.

[0370]An electrical response is determined for each chemo/electro-active
material upon exposure of the array to a gas mixture, and means for
determining the response include conductors interconnecting the sensor
materials. The conductors are in turn connected to electrical input and
output circuitry, including data acquisition and manipulation devices as
appropriate to measure and record a response exhibited by a sensor
material in the form of an electrical signal. The value of a response,
such as a measurement related to resistance, may be indicated by the size
of the signal. One or more signals may be generated by an array of
sensors as to each analyte component in the mixture, whether the analyte
is one or more individual gases and/or one or more subgroups of gases.

[0371]An electrical response is determined for each individual
chemo/electro-active material separately from that of each of the other
chemo/electro-active materials. This can be accomplished by accessing
each chemo/electro-active material with an electric current sequentially,
using a multiplexer to provide signals differentiated between one
material and another in, for example, the time domain or frequency
domain. It is consequently preferred that no chemo/electro-active
material be joined in a series circuit with any other such material. One
electrode, by which a current is passed to a chemo/electro-active
material, can nevertheless be laid out to have contact with more than one
material. An electrode may have contact with all, or fewer than all, of
the chemo/electro-active materials in an array. For example, if an array
has 12 chemo/electro-active materials, an electrode may have contact with
each member of a group of 2, 3, 4, 5 or 6 (or, optionally, more in each
instance) of the chemo/electro-active materials. The electrode will
preferably be laid out to permit an electrical current to be passed to
each member of such group of chemo/electro-active materials sequentially.

[0372]A conductor such as a printed circuit may be used to connect a
voltage source to a sensor material, and, when a voltage is applied
across the sensor material, a corresponding current is created through
the material. Although the voltage may be AC or DC, the magnitude of the
voltage will typically be held constant. The resulting current is
proportional to both the applied voltage and the resistance of the sensor
material. A response of the material in the form of either the current,
voltage or resistance may be determined, and means for doing so include
commercial analog circuit components such as precision resistors,
filtering capacitors and operational amplifiers (such as a OPA4340). As
voltage, current and resistance is each a known function of the other two
electrical properties, a known quantity for one property may be readily
converted to that of another.

[0373]Resistance may be determined, for example, in connection with the
digitization of an electrical response. Means for digitizing an
electrical response include an analog to digital (A/D) converter, as
known in the art, and may include, for example, electrical components and
circuitry that involve the operation of a comparator. An electrical
response in the form of a voltage signal, derived as described above as a
result of applying a voltage across a sensor material, is used as an
input to a comparator section (such as a LM339). The other input to the
comparator is driven by a linear ramp produced by charging a capacitor
using a constant current source configured from an operational amplifier
(such as a LT1014) and an external transistor (such as a PN2007a). The
ramp is controlled and monitored by a microcomputer (such as a
T89C51CC01). A second comparator section is also driven by the ramp
voltage, but is compared to a precise reference voltage. The
microcomputer captures the length of time from the start of the ramp to
the activation of the comparators to generate a signal based on the
counted time.

[0374]The resistance of the sensor material is then calculated, or
quantified as a value, by the microcomputer from the ratio of the time
signal derived from the voltage output of the material to a time signal
corresponding to a known look-up voltage and, ultimately, to the
resistance that is a function of the look-up voltage. A microprocessor
chip, such as a T89C51CC01, can be used for this function. The
microprocessor chip may also serve as means for determining a change in
the resistance of a sensor material by comparing a resistance, determined
as above, to a previously determined value of the resistance.

[0375]Electrical properties such as impedance or capacitance may be
determined, for example, by the use of circuitry components such as an
impedance meter, a capacitance meter or inductance meter.

[0376]Means for digitizing the temperature of an array of
chemo/electro-active materials can include, for example, components as
described above that convert a signal representative of a physical
property, state or condition of a temperature-measuring device to a
signal based on counted time.

[0377]In one embodiment, analysis of a multi-component gas mixture is
complete upon the generation of an electrical response, such as
resistance, in the manner described above. As a measurement of resistance
exhibited by a sensor material upon exposure to a gas mixture is a
function of the partial pressure within the mixture of one or more
component gases, the measured resistance provides useful information
about the composition of the gas mixture. The information may, for
example, indicate the presence or absence within the mixture of a
particular gas or subgroup of gases. In other embodiments, however, it
may be preferred to manipulate, or further manipulate, an electrical
response in the manner necessary to obtain information related to the
concentration within the mixture of one or more particular component
gases or subgroups of gases, or to calculate the actual concentration
within the mixture of one or more component gases or subgroups.

[0378]Means for obtaining information concerning the relative
concentration within the mixture of one or more individual component
gases and/or one or more subgroups of gases, or for detecting the
presence of, or calculating the actual concentration of, one or more
individual component gases and/or subgroups within the mixture, may
include a modeling algorithm that incorporates either a PLS (Projection
onto Latent Systems) model, a back-propagation neural network model, or a
combination of the two, along with signal pre-processing and output
post-processing. Signal pre-processing includes, but is not limited to,
such operations as principle component analyses, simple linear
transformations and scaling, logarithmic and natural logarithmic
transformations, differences of raw signal values (e.g., resistances),
and differences of logarithmic values. The algorithm contains a model
whose parameters have been previously determined, and that empirically
models the relationship between the pre-processed input signal and
information related to the gas concentration of the species of interest.
Output post-processing includes, but is not limited to, all of the
operations listed above, as well as their inverse operations.

[0379]The model is constructed using equations in which constants,
coefficients or other factors are derived from pre-determined values
characteristic of a precisely measured electrical response of an
individual sensor material to a particular individual gas or subgroup
expected to be present as a component in the mixture to be analyzed. The
equations may be constructed in any manner that takes temperature into
account as a value separate and apart from the electrical responses
exhibited by the sensor materials upon exposure to a gas mixture. Each
individual sensor material in the array differs from each of the other
sensors in its response to at least one of the component gases or
subgroups in the mixture, and these different responses of each of the
sensors is determined and used to construct the equations used in the
model.

[0380]A change of temperature in the array may be indicated by a change in
the quantified value of an electrical response characteristic, resistance
for example, of a sensor material. At a constant partial pressure in the
mixture of a gas of interest, the value of an electrical response
characteristic of a sensor material may vary with a change in temperature
of the array, and thus the material. This change in the value of an
electrical response characteristic may be measured for the purpose of
determining or measuring the extent of change of, and thus a value for,
temperature. The temperature of the array will be the same, or
substantially the same, as the temperature of the gas mixture unless the
array is being maintained at a pre-selected temperature by a heater
located on the substrate. If the array is being heated by a heater, the
temperature of the array will lie substantially in the range within which
the heater cycles on and off.

[0381]It is not required, but is preferred, that the measurement of
temperature be made independently of information related to the
compositional content of a gas mixture. This can be done by not using
sensors that provide compositional information for the additional purpose
of determining temperature, and, optionally, by connecting the
temperature measuring device in parallel circuitry with the sensor
materials, rather than in series. Means for measuring temperature include
a thermocouple or a pyrometer incorporated with an array of sensors. If
the temperature determining device is a thermistor, which is typically a
material that is not responsive to an analyte gas, the thermistor is
preferably made from a different material than the material from which
any of the gas sensors is made. Regardless of the method by which
temperature or change in temperature is determined, a temperature value
or a quantified change in temperature is a desirable input, preferably in
digitized form, from which an analysis of a mixture of gases and/or a
component therein may be performed.

[0382]In the method and apparatus of this invention, unlike various
prior-art technologies, there is no need to separate the component gases
of a mixture for purposes of performing an analysis, such as by a
membrane or electrolytic cell. There is also no need when performing an
analysis by means of this invention to employ a reference gas external to
the system, such as for the purpose of bringing a response or analytical
results back to a base line value. A value representative of a reference
state may, however, be used as a factor in an algorithm by which
information related to the composition of the gas mixture is determined.
With the exception of preliminary testing, during which a standardized
response value to be assigned to the exposure of each individual sensor
material to each individual analyte gas is determined, the sensor
materials are exposed only to the mixture in which an analyte gas and/or
subgroup is contained. The sensor materials are not exposed to any other
gas to obtain response values for comparison to those obtained from
exposure to the mixture containing an analyte. The analysis of the
mixture is therefore performed only from the electrical responses
obtained upon exposure of the chemo/electro-active materials to the
mixture containing the analyte. No information about an analyte gas
and/or subgroup is inferred by exposure of the sensor materials to any
gas other than the analyte itself as contained within the mixture.

[0383]This invention is therefore useful at the higher temperatures found
in automotive emission systems, typically in the range of from about
400° C. to about 1000° C. In addition to gasoline and
diesel internal combustion engines, however, there is a variety of other
combustion processes to which this invention could be applied, including
stack or burner emissions of all kinds such as resulting from chemical
manufacturing, electrical generation, waste incineration and air heating.
These applications require the detection of gases such as nitrogen
oxides, ammonia, carbon monoxide, hydrocarbons and oxygen at the ppm to
per cent levels, typically in a highly corrosive environment.

[0384]When the multi-component gas mixture comprises a nitrogen oxide, a
hydrocarbon, or both, or any of the other gases mentioned herein, the
apparatus may be used to determine the presence and/or concentration of a
nitrogen oxide and/or hydrocarbon in the multi-component gas mixture. The
apparatus may also be used to determine the presence and/or concentration
of any one or more to the other gases mentioned herein that may be
present in a multi-component gas mixture. For this purpose, the
electrical response, in the apparatus of this invention, of one or more
of a chemo/electro-active material that comprises M1Ox, a
chemo/electro-active material that comprises
M1aM2bOx, and a chemo/electro-active material
that comprises M1aM2bM3cOx, may be
related to one or more of the presence of a nitrogen oxide within the gas
mixture, the presence of a hydrocarbon within the gas mixture, the
collective concentration of all nitrogen oxides within the gas mixture,
and the concentration of a hydrocarbon within the gas mixture.

[0385]This invention therefore provides methods and apparatus for directly
sensing the presence and/or concentration of one or more gases in an
multi-component gas system, comprising an array of at least two
chemo/electro-active materials chosen to detect analyte gases or
subgroups of gases in a multi-component gas stream. The multi-component
gas system can be at essentially any temperature that is not so low or so
high that the sensor materials are degraded or the sensor apparatus
otherwise malfunctions. In one embodiment, the gas system may be at a
lower temperature such as room temperature (about 25° C.) or
elsewhere in the range of about 0° C. to less than about
100° C., whereas in other embodiments the gas mixture may at a
higher temperature such as in the range of about 400° C. to about
1000° C. or more. The gas mixture may therefore have a temperature
that is about 0° C. or more, about 100° C. or more, about
200° C. or more, about 300° C. or more, about 400°
C. or more, about 500° C. or more, about 600° C. or more,
about 700° C. or more, or about 800° C. or more, and yet is
less than about 1000° C., is less than about 900° C., is
less than about 800° C., is less than about 700° C., is
less than about 600° C., is less than about 500° C., is
less than about 400° C., is less than about 300° C., is
less than about 200° C., or is less than about 100° C.

[0386]In applications in which the gas mixture is above about 400°
C., the temperature of the sensor materials and the array may be
determined substantially only, and preferably is determined solely, by
the temperature of the gas mixture in which a gaseous analyst is
contained. This is typically a variable temperature. When
higher-temperature gases are being analyzed, it may be desirable to
provide a heater with the array to bring the sensor materials quickly to
a minimum temperature. Once the analysis has begun, however, the heater
(if used) is typically switched off, and no method is provided to
maintain the sensor materials at a preselected temperature. The
temperature of the sensor materials thus rises or falls to the same
extent that the temperature of the surrounding environment does. The
temperature of the surrounding environment, and thus the sensors and the
array, is typically determined by (or results from) substantially only
the temperature of the gas mixture to which the array is exposed.

[0387]In applications in which the gas mixture is below about 400°
C., it may be preferred to maintain the sensor materials and the array at
a preselected temperature of about 200° C. or above, and
preferably 400° C. or above. This preselected temperature may be
substantially constant, or preferably is constant. The preselected
temperature may also be about 500° C. or above, about 600°
C. or above, about 700° C. or above, about 800° C. or
above, about 900° C. or above, or about 1000° C. or above.
This may be conveniently done with a heater incorporated with the array,
in a manner as known in the art. If desired, a separate micro heater
means may be supplied for each separate chemo/electro-active material,
and any one or more of the materials may be heated to the same or a
different temperature. The temperature of the gas mixture in such case
may also be below about 300° C., below about 200° C., below
about 100° C., or below about 50° C. In these low
temperature application, the means for heating the chemo/electro-active
materials may be a voltage source that has a voltage in the range of
about 10-3 to about 10-6 volts. The substrate on which the
materials are placed may be made of a materials that is selected from one
or more of the group consisting of silicon, silicon carbide, silicon
nitride, and alumina containing a resistive dopant. Devices used in these
low temperature applications are often small enough to be held in the
human hand.

[0388]This heating technique is also applicable, however, to the analysis
of high temperature gases. When the temperature of the gas mixture is
above about 400° C., the sensor materials may nevertheless be
maintained by a heater at a constant or substantially constant
preselected temperature that is higher than the temperature of the gas
mixture. Such preselected temperature may be about 500° C. or
above, about 600° C. or above, about 700° C. or above,
about 800° C. or above, about 900° C. or above, or about
1000° C. or above. Should the temperature of the gas mixture
exceed the temperature pre-selected for the heater, the heater may be
switched off during such time. A temperature sensor will still be
employed, however, to measure the temperature of the gas mixture and
provide that value as an input to an algorithm by which information
related to the composition of the gas mixture is determined.

[0389]In summary, it may be seen that this invention provides means to
determine, measure and record responses exhibited by each of the
chemo/electro-active materials present in an array upon exposure to a gas
mixture. Any means that will determine, measure and record changes in
electrical properties can be used, such as a device that is capable of
measuring the change in AC impedance of the materials in response to the
concentration of adsorbed gas molecules at their surfaces. Other means
for determining electrical properties are suitable devices to measure,
for example, capacitance, voltage, current or DC resistance.
Alternatively a change in temperature of the sensing material may be
measured and recorded. The chemical sensing method and apparatus may
further provide means to measure or analyze a mixture and/or the detected
gases such that the presence of the gases are identified and/or their
concentrations are measured. These means can include instrumentation or
equipment that is capable, for example, of performing chemometrics,
neural networks or other pattern recognition techniques. The chemical
sensor apparatus will further comprise a housing for the array of
chemo/electro-active materials, the means for detecting, and means for
analyzing.

[0390]The device includes a substrate, an array of at least two
chemo/electro-active materials chosen to detect one or more predetermined
gases in a multi-component gas stream, and a means to detect changes in
electrical properties in each of the chemo/electro-active materials
present upon exposure to the gas system. The array of sensor materials
should be able to detect an analyte of interest despite competing
reactions caused by the presence of the several other components of a
multi-component mixture. For this purpose, this invention uses an array
or multiplicity of sensor materials, as described herein, each of which
has a different sensitivity for at least one of the gas components of the
mixture to be detected. A sensor that has the needed sensitivity, and
that can operate to generate the types of analytical measurements and
results described above, is obtained by selection of appropriate
compositions of materials from which the sensor is made. Various suitable
types of materials for this purpose are described above. The number of
sensors in the array is typically greater than or equal to the number of
individual gas components to be analyzed in the mixture.

[0391]Further description relevant to the apparatus of this invention,
uses for the apparatus and methods of using the apparatus may be found in
U.S. Provisional Application No. 60/370,445, filed Apr. 5, 2002, and U.S.
application Ser. No. 10/117,472, filed Apr. 5, 2002, each of which is
incorporated in its entirety as a part hereof for all purposes.